1. FIELD OF THE INVENTION
This invention relates in general to parabolic antennas and more particularly to a parabolic antenna of the type suitable for preventing a decrease in gain efficiency.
2. DESCRIPTION OF THE PRIOR ART
In receiving Direct Broadcasting Satellite (D.B.S.) communications by means of a parabolic antenna, efficiency of gain is generally affected to a great extent by physical or geometrical accuracy of a parabolic reflector surface and accuracy of the mounting position of a converter relative to the parabolic reflector. Therefore, in order to prevent a decrease in efficiency of gain, it is necessary to place the converter in position with high accuracy as described below and to maintain the geometrical accuracy of the parabolic reflector.
The accuracy of the mount position of the converter withh respect to the parabolic reflector generally depends on the wavelength of radio waves and the positional relationship therebetween must be set with high accuracy of the .+-..lambda./32 or less, where .lambda. is the wavelength of radio wave and .lambda./32 comes to about 0.78 mm for a wavelength of about 2.5 cm of a 12GH radio wave (KU-Band) used in the satellite broadcasting.
To clarify problems incountered in practice, a prior art parabolic antenna will first be explained with reference to FIGS. 5 and 6 respectively illustrating a side view and a rear view of the parabolic antenna fitted on an antenna support.
A parabolic reflector 20 has a rear structure which is integrally formed with a connecting block 21 and reinforcement ribs 26. An antenna fitting 22 fixed on an antenna support post 23 is assembled to the connecting block 21 by tightening screws so that the parabolic reflextor 20 is fixedly mounted to the support post 23. A converter 2 is rigidly connected to a flange of the parabolic reflector 20 by a mount arm 24 connected to a lower portion of the flange, and is adjusted for mount positioning by support bars 25 connected to right and left side edge portions of the parabolic reflector flange positioned about 1/3 of height of the parabolic reflector below the top thereof, in order to hold in position the mount arm 24 connected to the flange lower portion. Consequently, the converter 2 is supported uniformly relative to the parabolic reflector at three points to maintain positional accuracy. The support bars 25 must be curved so as not to intercept a radio wave incident to the parabolic reflector 20. When, in this parabolic antenna, a strong wind exerts a large wind pressure on the parabolic reflector 20, bending moments will be concentrated at the boundary between the connecting block 21 and part of the rear structure of parabolic reflector 20 surrounding the block 21, and the parabolic reflector 20 tends to be distorted permanently about the boundary. In addition, because of the connection of the converter 2 to the parabolic reflector by means of the converter mount arm 24 rigidly secured to the flange lower portion, bending moments are also concentrated to a lower part of the flange, thereby aggravating the tendency of the parabolic reflector toward permanent distortion. The distortion leading to permanent deformation caused in the parabolic reflector will degrade the efficiency of gain.
This disadvantageous distortion and permanent deformation of the elastic parabolic reflector caused by a strong wind can be mitigated in the prior art by increasing the thickness of the parabolic reflector 20 and adding the reinforcement ribs 26 for support of the parabolic reflector rear structure over a wide area so as to increase mechanical strength as shown in FIGS. 5 and 6. This expedient however requires a large amount of materials to be used and results in a complicated structure.
For assuring the accuracy of mounting position of the converter 2 relative to the parabolic reflector 20, on the other hand, the three-point support is adopted as described previously wherein the mount arm 24 of the converter 24 rigidly connected to the flange of the parabolic reflector is supported by the converter arm support bars 25 extending from the both side edge portions of the parabolic reflector flange. Disadvantageously, the converter arm support bars 25 even though not extending across the effective area of the parabolic reflector tend to cause irregular reflection of the radio wavd which in turn becomes an additional factor of degrading the efficiency of gain.
Another prior art reflector as shown in FIGS. 7 and 8 is configured as will be described below to avoid deformation of a parabolic reflector 20 due to pressure of a strong wind exerting on a converter 2 and on a converter mount arm 24. More particularly, bending moments attributable to the wind pressure on the converter 2 can be applied to a fitting 22 without interferring with the parabolic reflector 20 by connecting the converter mount arm 24 directly to the fitting 22 or to a parabolic reflector fixture 27 integral with a connecting block 21. With this construction, however, to obtain highly accurate mounting position of the converter 2 relative to the parabolic reflector 20, positional accuracy between the parabolic reflector 20 and parabolic reflector fixture 27, positional accuracy between the parabolic reflector fixture 27 and fitting 22, positional accuracy between the fitting 22 and converter mount arm 24 and mount accuracy of the converter mount arm 24 to the converter 2 are all required to be extremely high. To meet such a requirement, the individual components must be highly accurate, especially, mount surfaces of the individual components must be machine accurately in the extreme and besides the fixture 27 with connecting block 21 is required to be large sized structurally.
As will be seen from the above, the conventional parabolic antenna needs parts with very high accuracy, and therefore it has been very difficult to significantly improve the efficiency of gain.